1. Field of Invention
The present invention generally relates to a control system for an internal combustion engine in which a fresh air blowby amount passing from an intake passage through to an exhaust passage may be estimated based on a measurement of an intake air amount in the intake passage.
2. Description of Related Art
A conventional technology for reducing a residual gas amount within in a cylinder is to make effective use of a scavenging effect due to valve overlap, in which an amount of valve overlap is controlled according to an estimated amount of fresh air blowby which passes from an intake passage into an exhaust passage. The fresh air blow-by is estimated based on an oxygen concentration detected by an air-fuel ratio sensor disposed in the exhaust passage.
In conventional systems, however, when estimating the fresh air blowby from an intake passage into an exhaust passage based on the oxygen concentration in the exhaust passage, the oxygen concentration may vary according to the fuel injection amount. In addition the oxygen concentration may be subject to a so-called after-burn phenomenon which takes place in the exhaust passage with a fuel-rich exhaust gas reacting with blowby air. Therefore, there remains a problem of not always being able to estimate the amount of fresh air blowby with sufficient accuracy. Note that the fresh air blowby amount may alternatively be referred to below as a scavenging amount.